The present invention pertains to an improved lithographic ink composition and to a process for making the improved composition. In particular, the present invention enables the production of an ink composition having a stable pH in the printing process. pH stability is highly desirable, as without pH stability smudging of the resulting image will result, due to the acidic fountain solution becoming more basic at the region in which the fountain solution interfaces with the ink. The pH sensitivity is aggravated when an ink comprising a Lithol Rubine pigment is employed in the printing process.
The closest prior art of which applicants are aware is PCT International Publication No. WO 84/02142, published June 7, 1984, assigned to Inmont Corporation of Clifton, N.J. Unlike the invention described herein, this publication teaches the incorporation of water-soluble buffers into the fountain solution, not the ink. The Inmont publication does not teach the incorporation of a buffer into a nonaqueous ink. This difference is believed to be critical as it is believed that addition of the buffer to the fountain solution significantly decreases the availability of buffer to the interface between the ink and the fountain solution, as is explained below.
In the art of lithographic printing, an oleophilic, oil-based printing ink composition is mixed with a water-based, oleophobic fountain solution. Immediately after the mixing of the oil ink with the fountain solution, a water-in-oil emulsion is created by subjecting the combination of ink and fountain solution to severe turbulence and shear. Shortly after the water-in-oil emulsion is made, the emulsion contacts a printing plate which has both oleophilic and oleophobic surfaces thereon. If the pH of the fountain solution is not within the proper range, image definition at the interface between oleophilic and oleophobic regions is blurred, reducing image resolution and occasionally causing smudging.
In the process of the prior art, a water-soluble buffer is mixed with a fountain solution. During the printing process, the buffered fountain solution is combined with and emulsified within an oleophilic ink. A water-in-oil emulsion is formed thereby. The buffer, preferring the aqueous fountain solution, remains within the fountain solution. Since there is no significant oil-solubility for any significant exchange of buffer from the hydrophilic fountain solution to the oleophilic ink, the result is that virtually all of the buffer remains within the fountain solution. Thus, at the interface between the fountain solution and the ink, the availability of buffer at the interface region is "one-sided." In other words, the only buffer reaching the interface to control pH drift is buffer supplied from the fountain solution side of the interface.
The present invention utilizes an oil-soluble buffer. The buffer is added to the oleophilic ink. The ink and fountain solution are then combined and a water-in-oil emulsion is formed, with the resulting emulsion being used immediately thereafter in the printing process. The vast majority of buffer remains in the oleophilic ink phase. A minor proportion of the acid portion of the buffer (i.e. that which is disassociated) migrates into the fountain solution. In comparison with the prior art, there is a greater balance between the amount of buffer in the ink and the amount of buffer in the fountain solution. The result of this balance is a greater availability of buffer to the interface. This greater availability of buffer to the interface provides greater pH control (i.e. less pH drift) during the resulting printing process.
The present invention is distinguishable from the reference in that the present invention employs an ink composition in which the buffer is added to the ink, as opposed to being added to the fountain solution.